Journal of Arid Environments 103 (2014) 80e84 Contents lists available at ScienceDirect Journal of Arid Environments journal homepage: www.elsevier.com/locate/jaridenv Distribution of arid-dwelling land snails according to dryness Gregorio Moreno-Rueda Departamento de Zoología, Universidad de Granada, ES-18071 Granada, Spain article info abstract Article history: Although land snails are hydrophilic animals, several species inhabit arid or semi-arid environments. Received 13 September 2013 Here, I hypothesize that, for arid-dwelling land snails, both relatively moist environments and extreme Received in revised form arid zones, within their distribution ranges, should be disadvantageous. Therefore, arid-dwelling land 15 January 2014 snails should show maximal probability of presence and maximal abundances at intermediate levels of Accepted 21 January 2014 aridity. I tested this hypothesis with two land-snails from Sierra Elvira mountain range (SE Spain), Available online Sphincterochila candidissima and Iberus gualterianus. Given that environmental variables as well as snail distribution showed spatial autocorrelation, I performed spatially explicit models, specifically simulta- Keywords: Autoregressive models neous auto-regressions (SAR). The results supported the hypothesis, with the distribution of Habitat selection S. candidissima and the abundance of I. gualterianus following a concave-down relationship with aridity. Land snails Moreover, both species were less abundant as elevation increased, and I. gualterianus showed a positive Mountain association with rocky surface. Therefore, this study highlights that, in arid environments, arid-dwelling Spatial autocorrelation land snails show maximal abundance and probability of presence at intermediate aridity levels. Although Spatially explicit models the reasons explaining why extreme aridity values limit the abundance and distribution of land snails are well detailed, it remains intriguing why these snails decrease in abundance when moisture increases. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction semi-arid Sierra Elvira mountain (SE Spain), in contrast to moist sites, shows higher abundances and diversity of land snails on its Arid environments present characteristically low moisture and drier, southern slope (Moreno-Rueda, 2002). Although dry zones scarce vegetation (Cox and Moore, 2005). Consequently, primary offer a clearly restrictive environment for land snails (also see productivity is very low in arid environments (Hawkins et al., Tryjanowski and Koralewska-Batura, 2000), wet zones may also be 2003), and this imposes restrictive living conditions on animals restrictive in some aspects, especially for animals not adapted to (Pianka, 2000). Arid environments are especially restrictive for moist environments. Therefore, the dispersion of arid-dwelling hydrophilic animals, despite that several hydrophilic taxa inhabit land snails to moister zones may be limited by a number of fac- arid environments. For example, terrestrial gastropods are very tors. For example, parasites are more abundant in wet zones (Moyer hydrophilic animals, since they have a permeable skin and undergo et al., 2002). Given that species richness is higher in more pro- high rates of dehydration (Luchtel and Deyrup-Olsen, 2001; Prior, ductive zones (e.g. Moreno-Rueda and Pizarro, 2009), then more 1985). In fact, in temperate realms, their distribution is strongly productive zones (i.e. moister zones) should harbour more preda- affected by moisture, and a greater number of individuals are tors and competitors. Competition with other species or subspecies typically found in moist environments than in dry environments may limit the distribution of land snails (Moreno-Rueda, 2006b). In (Martin and Sommer, 2004). Nevertheless, several snail species fact, climatic selection strongly influences land snail distribution inhabit arid zones, where they use behavioural, physiological, and/ (Cowie, 1990; Cowie and Jones, 1985; Johnson, 2011). or morphological adaptations to minimise the risk of dehydration In accordance with this reasoning, I predict that in an arid or (Arad et al., 1989; Giokas et al., 2005; Moreno-Rueda, 2007). semi-arid zone, such as Sierra Elvira mountain range, the abun- As mentioned above, in relatively moist zones, land snails are dance and probability of detecting arid-dwelling land snails should typically more abundant in wetter zones. Therefore, it might be show a concave-down relationship with aridity. In a cline of aridity, concluded that in arid or semi-arid zones, such as the Mediterra- both extremes should impose restrictive situations for land snails nean region, arid-dwelling land snail distributions increase with by the aforementioned conditions, with their abundances maxi- moisture, even more markedly than in wet realms. However, the mized at intermediate aridity values. I test this prediction with the two main species of land snails in Sierra Elvira: Sphincterochila candidissima and Iberus gualterianus (Moreno-Rueda, 2002). In the present study, I measured in detail a number of environmental E-mail address: [email protected]. 0140-1963/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jaridenv.2014.01.006 G. Moreno-Rueda / Journal of Arid Environments 103 (2014) 80e84 81 variables, and statistically tested whether the abundance values randomly distributed on the mountain (Fig. 1). Quadrats were and probabilities of detection of the two snails show a concave- cordoned off, and I carefully searched for snails by the complete down relationship with plant cover, as an inverse indicator of quadrat, especially scrutinizing rock crevices as well as searching micro-geographic aridity. inside and under scrubby vegetation, where these snails are frequently sheltered (Moreno-Rueda, 2007). Live specimens as well 2. Methods as empty shells were recorded. Since snails inhabit a low- productive environment, they are found at very low densities 2.1. Study species (Moreno-Rueda and Collantes-Martín, 2007; Moreno-Rueda and Pizarro, 2007). Therefore, I estimated relative abundance according S. candidissima (Gastropoda; Sphincterochilidae) is widespread to total live plus dead snails found. Previously, I tested whether this in the west side of the Mediterranean basin (Fechter and Falkner, is a good estimation of relative abundance of live snails. For both 1993). All the species in Sphincterochilidae are well adapted to species, density of live animals was significantly correlated with desert and subdesert environments (Arad et al., 1989; Shachak, that of dead snails (for I. gualterianus: rs ¼ 0.578, P < 0.001; for 1981; Yom-Tov, 1971). On the other hand, the genus Iberus (Gas- S. candidissima: rs ¼ 0.549, P < 0.001; n ¼ 70 quadrats). Slopes tropoda; Helicidae) is endemic to Spain (García San Nicolás, 1957) might encourage the accumulation of empty shells in lower flat and typically inhabits zones with Mediterranean climate (Ruiz Ruiz areas (Baur et al., 1997). For this reason, I measured the slope angle et al., 2006). I. gualterianus inhabits mainly arid and semi-arid of the quadrat with an inclinometer at 16 points homogeneously zones such as the mountain range sampled in the present study distributed, and estimated the average. The density of empty shells (Moreno-Rueda, 2011). was uncorrelated with average inclination of the quadrat for both species (I. gualterianus: rs ¼ 0.189, P ¼ 0.117; S. candidissima: 2.2. Study area rs ¼ 0.143, P ¼ 0.239). Therefore, I considered a species to be present in a quadrat when I found at least one shell in the quadrat. The Sierra Elvira (SE Spain; 37 150 N, 3 400 W), is a small karstic relative abundance was estimated as the total of empty shells plus sierra within an elevational range of 600e1100 m a.s.l. It is char- live individuals found in the quadrat. acterized as having a Mediterranean climate with up five months of Regarding the environmental variables, I measured: (1) Plant drought (Moreno-Rueda et al., 2009). The study area shows a cover, which was measured by subdividing the quadrat in 225 mosaic of habitats composed mainly of scrubland of Quercus coc- squares of 20 Â 20 cm. For each square, I recorded whether it was cifera, Juniperus oxycedrus, Stipa tenacissima, Cistus ssp. and Ros- mainly (>50%) covered by vegetation or not. The percentage of marinus officinalis, alternating with small patches of grasslands and cover was calculated as number of squares covered by vegetation bare soil. divided by 2.25. Aridity of the quadrats was approached as the absence of vegetation, estimated as 100 minus plant cover, that is, 2.3. Sampling the percentage of the quadrat not covered by vegetation. (2) Elevation, with the use of a GPS device. Elevation was measured The sampling was performed between 2002 and 2004, during because previous studies show that it affects the presence at least of October and November, when detectability of snails is at its peak I. gualterianus (Moreno-Rueda, 2006c). (3) Rocky substrate. In the (Moreno-Rueda and Collantes-Martín, 2007; Moreno-Rueda and study area, substrate may be divided basically into bare soil and Pizarro, 2007). For the measurement of snail distribution and rocky substrate. I measured the type of substrate, because it has environmental variables, I studied 70 quadrats of 9 m2 (3 Â 3m) been proved to be important for the distribution of I. gualterianus Fig. 1. Geographic location of study quadrats in Sierra Elvira mountain range (SE Spain). White indicates quadrats in which